Not necessary, because even if the emotion the subjects agreed on was not the emotion the person photographed was in fact feeling, that person could still have been expressing that emotion. What matters is the emotion the person expressed, not what they actually felt internally.

Not necessary, because the argument isn’t trying to explain the cause of someone’s emotional disposition (how they tend to feel). The argument concerns what explains why people agreed on the emotion being expressed in certain photographs. How the people who were looking at the photographs felt emotionally is irrelevant.

Not necessary, because (C) would actually undermine the argument if assumed. The author assumes that the behavior of the people in the study was NOT the product of culture. The author does not have to assume that anything IS the product of culture.

Necessary, because if it were not true — if the fact that a behavior is common to people of widely different cultures does NOT indicate probable genetic predisposition for that behavior — then we have no reason to think that the fact people agreed on the emotions shown in the photos indicates a genetic predisposition. The author believes that the universal agreement, between people of widely different cultures, on the emotions displayed in the photos is evidence of a genetic origin for the identification of those emotions.

Not necessary, because the negation would arguably help the argument. If the people in the photos were ALL from the same culture, and yet everyone from the widely disparate cultures still identified the same emotions for each photo, that tends to suggest culture may not have played a role in the identification. Because even someone who did not share the culture of the person in the photos still agreed on the emotion displayed.